The possible failure of an electric motor in systems of the above kind may have different effects. In some systems the failed motor may simply remain stopped without participating in the further driving. In other systems the failed motor can remain in the drive train and be driven by the auxiliary or reserve motor. Under certain circumstances, for example, when the motor winding is burned through or when the bearings have become defective, the failed motor may block the drive train. In both instances, namely when the rotor of the motor is still capable of rotating, or when the rotor is blocked, it is necessary to positively lock the failed motor by means of a brake which requires the use of a differential gear drive as part of the redundant mechanism. Such a differential gear drive is necessary because the reserve motor can drive the system, for example an outrigger to be extended, only if the failed motor is properly blocked. Such a system requires the use of sensors, magnetic brakes, and switches which in turn must be provided as redundant features and these redundant features must be properly controllable. However, the use of additional redundant features also has its disadvantages because it results in increased weight and, under certain circumstances, it may even reduce the functional reliability of the entire system.